Many patients that suffer from congestive heart failure (CHF) develop a wide QRS complex resulting from a delayed activation of one of the ventricles in the heart, and inter- and/or intraventricular electrical-mechanical dysynchrony. This ventricular “dysynchrony” may be caused by dilation of the heart, which disrupts the conductive pathways and interferes with depolarization sequences. Ventricular dysynchrony may worsen heart failure symptoms.
In a classic case of ventricular dysynchrony, the right ventricle of the heart activates first, and the left ventricle activates at a later time. Delayed activation of the left ventricle may be caused by a particular disruption of the conductive pathways of the heart, referred to as a left bundle branch block (LBBB). A patient who has LBBB often experiences a reduction in cardiac output because of dysynchronous ventricular contraction. Moreover, in the case of LBBB, different regions within the left ventricle may not contract together in a coordinated fashion, further reducing cardiac output.
Patients having a wide QRS complex or having inter- and/or intraventricular electrical-mechanical dysynchrony often are treated with an implanted medical device, such as a pacemaker, that paces both ventricles. The implanted medical device senses or paces atrial contractions, waits a predetermined time (or atrioventricular (AV) delay) after each sensed or paced atrial contraction, and then paces both ventricles. The ventricles may be paced simultaneously, or one ventricle may be paced before another. This biventricular pacing is often referred to as cardiac resynchronization.